(1) Field of the Invention
The present invention relates to a wireless communication system, and in particular to a wireless paging system, including a transmitting apparatus and a plurality of wireless terminals.
(2) Description of the Related Art
In wireless paging systems, paging signals are transmitted from a wireless transmission base station to page respective pager terminals which respective persons to be paged carry. Since each pager terminal is usually energized by a battery contained therein, the pager terminal is preferably controlled to be in a sleep mode when the pager terminal is not receiving a paging signal addressing the pager terminal. In the sleep mode, power from the battery is supplied to respective portions including a wireless signal receiving portion of the pager terminal intermittently for a short time with a predetermined cycle, while the power is constantly supplied to a main control portion. During the above short time in each cycle, the pager terminal tries to detect a preamble signal, and, when the preamble signal is detected, the pager terminal goes into a working mode to try to detect a frame synchronization pattern and an identification number, which is assigned in advance to the pager terminal. When both the frame synchronization pattern and the identification number of the pager terminal are not detected, the pager terminal gets back to the sleep mode.
In addition, to save power in the battery, the pager terminals have a function to automatically go into the sleep mode when no meaningful signal is received after receiving a paging signal addressing the pager terminal.
Conventionally, a batch of accumulated paging signals are asynchronously transmitted from a transmission apparatus in the base station. In a first known wireless paging system, the transmission apparatus is off in intervals between the transmission of the batches of the accumulated paging signals. In a second known wireless paging system, the transmission apparatus is not off and transmits a signal representing an alternating-pattern 101010 . . . with a transmission rate which is different from a transmission rate in the above transmission of the paging signals.
In the above first wireless paging system, when the transmission apparatus is off, no meaningful signal (noise only) is detected by the pager terminals, and therefore, the pager terminals goes into the sleep mode. In the above second wireless paging system, when the above signal representing the alternating pattern of a different transmission rate is transmitted from the transmission apparatus, the signal is sampled by the pager terminals at a timing of clock signals synchronized with the above paging signals, and therefore, the sampled signal values form random bit sequences. When the pager terminals detect such random bit sequences, generally, the pager terminal detects neither the frame synchronization pattern nor the identification number, and therefore, conventionally, it was believed that the pager terminals goes into the sleep mode.
However, In the above first wireless paging system, the transmission apparatus must be frequently switched on and off for the beginning and ending of the transmission of the batch of the paging signals. This frequent switching of the transmission apparatus seriously degrades transmission devices in the transmission apparatus. In the second wireless paging system, it is reported that the above random bit sequences sometimes accidentally coincide with the frame synchronization pattern and one of the identification numbers assigned to the subscribers of the wireless paging system. When such a coincidence occurs, the above subscriber is paged by error.